CI: add uncrustify config.

Run in the repo's root directory with for example

```
$ uncrustify -c data/uncrustify.cfg src/**/*.c
```
2 jobs for wip/nielsdg/uncrustify in 3 minutes and 20 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #167498
build-mutter

00:00:52

 
  Test
passed #167499
test-mutter

00:02:28